CamoSpecs Online is usually a good source, but alas, doesn't mention them by name. They do mention the Wolf Spider Battalion, spiritual heirs of the Black Widows, as being in black with red trim.
And it's worth noting that the new Classic images on the cover of Combat Manual: Mercenaries are reported as being Black Widows - the Warhammer being the Black Widow herself - and they're in black with red trim. And - perhaps more definitively - the painted mini on page 104 of CM:M representing the Black Widow company is, you guessed it, black with red trim.
My copy of the FASA-era Black Widow scenario pack is at home, so I can't check it, but I'd say you can go with the above with confidence.
Cheers,
W.